This invention relates to oxidation of iron-chromium-aluminum alloy foil to form thereon an oxide layer characterized by multitudinous whiskers. More particularly, this invention relates to a preparatory purification-treatment for foil composed of contaminated alloy to allow subsequent growth of the whiskers.
Aluminum-containing ferritic stainless steel has properties including corrosion resistance that render it particularly useful for high temperature applications, for example, as a substrate in an automotive catalytic converter. A typical steel comprises 15 to 25 weight percent chromium, 3 to 6 weight percent aluminum and the balance mainly iron. When exposed to oxygen at elevated temperatures, this iron-chromium-aluminum alloy forms a surface alumina layer that protects the underlying metal against further corrosion. The alloy may contain a small amount of an agent such as yttrium or cerium that promotes adherence of the oxide to the metal and thereby further improves high temperature corrosion resistance.
The surface of the protective oxide layer typically formed on iron-chromium-aluminum alloy is relatively smooth. However, under certain conditions, an oxide layer is formed that is characterized by multitudinous whiskers. The whiskers substantially improve bonding of an applied coating. U.S. Pat. No. 4,331,631, issued to Chapman et al in 1982, describes growth of the whiskers on foil formed by a metal peeling process. U.S. Pat. No. 4,318,828, issued to Chapman in 1982, describes a two-step oxidation treatment which is particularly useful for growing the whiskers on cold-rolled foil. In the two-step treatment, the foil is initially heated for a brief time on the order of a few seconds in a low oxygen atmosphere to form a precursor oxide film. Thereafter, the whiskers are grown by heating in air for several hours at a suitable temperature.
It has been found that some iron-chromium-aluminum alloy foil does not grow the desired multitudinous whiskers even when oxidized under preferred conditions for doing so. In particular, difficulty has been encountered in growing whiskers on commercial cold-rolled stock prepared from large heats. I have now found that this inability to grow the whiskers is related to magnesium impurity in the metal. In a typical example of contaminated foil, magnesium may be present in an amount on the order of 0.01 weight percent. Magnesium may be introduced in the constituent metals or in an agent for deoxidation or desulfurization. Another potential source is the refractory lining of the crucible or other vessel in which the alloy is melted. This lining is predominantly an inert ceramic such as alumina, but may contain a small amount of magnesium oxide. In preparing a large heat, the metal may reside in contact with the refractory for an extended time, during which magnesium may leech into the melt. In any event, I have found that the presence of magnesium in an amount greater than about 0.002 weight percent noticeably inhibits growth of the desired whiskers.
Therefore, it is an object of this invention to provide a method for treating aluminum-containing stainless steel foil comprising magnesium impurity in an amount sufficient to inhibit oxide whisker formation, which method selectively removes magnesium from the alloy and thereby permits multitudinous whiskers to be subsequently grown thereon. One feature of this invention is that the treatment is carried out on the solid steel to purify the alloy without physically altering the foil or requiring change to processes or equipment for preparing the alloy or manufacturing the foil. Indeed, the method is particularly useful when applied to contaminated foil stock to allow whisker growth on stock that otherwise would not be suitable. Furthermore, the treatment of the iron-chromium-aluminum alloy, which alloy may optionally contain an oxide adherence agent such as yttrium or cerium, removes unwanted magnesium without altering the composition of the base alloy or adversely affecting the desired high temperature properties of the steel.